AJP - Regulatory, Integrative and Comparative Physiology, Vol 257, Issue 3 461-R472, Copyright © 1989 by American Physiological Society
Energetics of sodium-coupled active transport mechanisms in invertebrate epithelia
G. A. Gerencser and B. R. Stevens
Department of Physiology, College of Medicine, University of Florida, Gainesville 32610.
The Na+ gradient has been implicated as the sole or primary energy source
for accumulative transport of organic solutes (e.g., sugars, amino acids)
across the mucosal membrane of a variety of epithelial cells. A basic
question concerning the Na+-coupled transport process in epithelia is
whether the energy available from the transmucosal Na+ electrochemical
difference is sufficient to sustain an accumulated organic solute
steady-state level. Measurements of Na+ activities, with Na+-sensitive
microelectrodes, gave accurate estimates of the Na+ electrochemical
potential difference across the mucosal membrane of Aplysia californica
gut. The results suggest that the transmucosal Na+ gradient can furnish
sufficient energy to sustain the observed intracellular levels of the
cotransported species. Many other species sustain large
intracellular-extracellular gradients (less than 10(6):1) of free solutes.
Theoretical models suggest that secondary active transport mechanisms in
these epithelia operate by energetic multiple coupling to the Na+
electrochemical gradient; coupling coefficients of approximately 3 may
represent an evolutionary optimization of these epithelial cotransporters.
To properly investigate the cotransport mechanisms and energetics in
invertebrate membranes, prototype mammalian vesicle experiments should be
extended to the invertebrate laboratory.
